Volume 604, August 2017
|Number of page(s)||10|
|Section||Galactic structure, stellar clusters and populations|
|Published online||01 August 2017|
Spectroscopic study of the elusive globular cluster ESO452-SC11 and its surroundings ⋆
1 Department of Physics, Lancaster University, LA1 4YB, Lancaster, UK
2 Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
3 Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
4 Saint Martin’s University, Old Main, 5000 Abbey Way SE, Lacey, WA 98503, USA
Received: 14 March 2017
Accepted: 24 April 2017
Globular clusters (GCs) have long been recognized as being amongst the oldest objects in the Galaxy. As such, they have the potential of playing a pivotal role in deciphering the Milky Way’s early history. Here we present the first spectroscopic study of the low-mass system ESO452-SC11 using the AAOmega multifibre spectrograph at medium resolution. Given the stellar sparsity of this object and the high degree of foreground contamination due to its location toward the Galactic bulge, very few details are known for this cluster – there is no consensus, for instance, about its age, metallicity, or its association with the disk or bulge. We identify five member candidates based on common radial velocity, calcium-triplet metallicity, and position within the GC. Using spectral synthesis, the measurement of accurate Fe-abundances from Fe-lines, and abundances of several α-, Fe-peak, and neutron-capture elements (Si, Ca, Ti,Cr, Co, Ni, Sr, and Eu) is carried out, albeit with large uncertainties. We find that two of the five cluster candidates are likely non-members, as they have deviating iron abundances and [α/Fe] ratios. The cluster mean heliocentric velocity is 19 ± 2 km s-1 with a velocity dispersion of 2.8 ± 3.4 km s-1, a low value in line with its sparse nature and low mass. The mean Fe-abundance from spectral fitting is −0.88 ± 0.03 dex, where the spread is driven by observational errors. Furthermore, the α-elements of the GC candidates are marginally lower than expected for the bulge at similar metallicities. As spectra of hundreds of stars were collected in a 2-degree field centered on ESO452-SC11, a detailed abundance study of the surrounding field was also enabled. The majority of the non-members have slightly higher [α/Fe] ratios, in line with the typical nearby bulge population. A subset of the spectra with measured Fe-peak abundance ratios shows a large scatter around solar values, albeit with large uncertainties. Furthermore, our study provides the first systematic measurements of strontium abundances in a Galactic bulge GC. Here, the Eu and Sr abundances of the GC candidates are broadly consistent with a disk or bulge association. Recent proper motions and our orbital calculations place ESO452 on an elliptical orbit in the central 3 kpc of the Milky Way, establishing a firm connection with the bulge. Finally, while the radial velocities and preferential position of a dozen of stars outside the GC radius appear to imply the presence of extra-tidal stars, their significantly different chemical composition refutes this hypothesis.
Key words: stars: abundances / Galaxy: abundances / Galaxy: structure / globular clusters: individual: ESO452-SC11 / Galaxy: disk / Galaxy: bulge
Full Tables 2–5 are available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (220.127.116.11) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/604/A41
© ESO, 2017
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